I still do not manage to move my cursor in the input string, even after following the advice from the QTBUG-15942 discussion. For the rest, everything works fine. I nevertheless think that the bug can be closed. It now rather looks like a missing feature to me.

The cursor position bug does not depend on TeXmacs. I've checked and the input method (with iBus) does not pass any cursor information to the application. This seems to be a bug of the input method or of the Qt layer in between.

A current bug under Ubunto is that the cursor keys do move the cursor inside the pre-edit string, but the current position is not correctly sent to TeXmacs. For instance, when typing
noro left left x,
I indeed get the x on the left in my pre-edit "window",
but the cursor remains on the right (as confirmed by selecting
Keyboard in the Debug menu.